Protein trafficking

Coated vesicles are vesicles whose membrane
has on its surface a layer of a protein such as clathrin, cop-I or
COP-II.Endocytosis is process by which
proteins at the surface of the cell are internalized, being transported
into the cell within membranous vesicles.Exocytosis is the process of secreting proteins from
a cell into the medium, by transport in membranous vesicles from the
endoplasmic reticulum, through the Golgi, to storage vesicles, and finally
(upon a regulatory signal) through the plasma membrane.Retrograde transport describes movement of proteins
in the reverse direction in the reticuloendothelial system, typically from
Golgi to endoplasmic reticulum.

Figure 25.8 Proteins are transported in coated
vesicles. Constitutive (bulk flow) transport from ER through the Golgi
takes place by COP-coated vesicles. Clathrin-coated vesicles are used for
both regulated exocytosis and endocytosis.

Figure 25.2 Vesicles are released when they bud
from a donor compartment and are surrounded by coat proteins (left).
During fusion, the coated vesicle binds to a target compartment, is
uncoated, and fuses with the target membrane, releasing its contents
(right).